Can you not just buy a phone & get a SIM-only contract in the US?
1) Different technologies need different phones. For example, an iPhone won't get HSPA+ on T-Mobile because T-Mobile uses AWS. Similarly, you can't use a GSM phone on a CDMA network, and vice versa
2) Except for T-Mobile, you pay the same both off and on contract. This is probably the biggest reason that it doesn't make sense. Even T-Mobile only gives you a discounted price after you've been on the contract. If you are lucky enough to be in such a state, and like T-Mobile's phones (or don't mind living with EDGE data), then it's great! Else, it doesn't make much financial sense.
To answer his question: Yes, there are many "SIM-only" (commonly called "prepaid" in the US) options. AT&T has the GoPhone thing, and T-Mobile has a couple of different prepaid options.
You can get smartphone data with both AT&T and T-Mobile's prepaid. A contract-free iPhone (3GS/4/4S) on AT&T GoPhone is a pretty good deal right now. Using an iPhone on GoPhone doesn't require that it be unlocked, as it uses standard AT&T (micro-)SIMS.
The various CDMA carriers and the MVNOs operating off of it has similar deals, though they're not as cost-effective and there's a choice of no or expensive data options.
However, the implication is that by buying a SIM (which can also be postpaid. It is not strictly prepaid only) and a phone, you get the choice of phone and service. This may be true in Europe. It is certainly true in India, where people routinely swap SIM's.
It is not true in the U.S. There is largely no benefit to buying service and a SIM separately. Yes, prepaid is cheaper, and I forgot about that. But if you look at an apples to apples comparison of plans, there is absolutely no reason to buy postpaid off-contract when you pay the same amount regardless.
People hate paying $600 for an iPhone when they can just put it on credit and never do the math.